Gel Formation and Structure of Junctions in Poly(vinyl alcohol)–Water Systems

Abstract

The main purpose of this investigation was to elucidate the mechanism of gelation of poly(vinyl alcohol)-water systems and the structure of junctions in gel networks. Changes of gel rigidities were measured as a function of gelling time at the temperatures of 30-80°C for four samples of different tacticities, and the numbers of junctions in the gel were calculated using the theory of rubber elasticity. The formation of junctions can be treated as the binary association of free cross-linking sequences with the activation energy of 13 kcal/mol. The study of the correlation between gelling temperatures and melting temperatures of gels shows that the junctions formed at a given temperature should be treated as multitudinous in structure and the copolymeric character of atactic poly(vinyl alcohol) chains should be considered. The structures of junctions for various gelling temperatures are discussed, assuming that cross-linking loci in poly(vinyl alcohol) chains are syndiotactic sequences of different sequence lengths.

Structure and dynamics of poly(vinyl alcohol) gels in mixtures of dimethyl sulfoxide and water

“…With respect to crystallization, PVA is regarded as a copolymer of different stereoisomers, and only the syndiotactic-rich sequences are available for crystallization. 36,37 According to the theory of crystallization in copolymers proposed by Flory, 38 sequences longer than the minimum stable crystallite length at equilibrium, which increase with temperature, can convert to crystallites. The number of syndiotactic sequences longer than the minimum length per chain, or the number of crystallites, therefore decreases with increasing temperature.…”

1H NMR Self-Diffusion Study of Morphology and Structure of Polyvinyl Alcohol Cryogels

“…We used the commercial crystallized bovine serum albumin, BSA (Sigma-Aldrich), without further purification. Deuterium oxide, D 2 O (99.9% D), and DMSO-d 6 (99.8% D) (ISOTEC Inc.) were used in the NMR experiments.…”

“…A Bruker AMX 300 WB spectrometer, equipped with a custom-built 10-mm diffusion probe from Cryomagnet Systems, Indianapolis, was used, and the pulsed-gradient stimulated echo sequence (35) was em-ployed to determine the self-diffusion of the polymer, water, and DMSO-d 6 . All experiments were based on the three rf-pulse stimulated echo-pulsed gradient spin-echo sequence using an eight rf-pulse phase cycle and five equally spaced gradient pulses (the first three serve to achieve a first-order instrumental steadystate situation prior to the start of the actual experiment).…”

“…A concentrated solution of BSA in D 2 O (or in mixture of D 2 O-DMSO-d 6 ) was added to the cooled PVA solution up to total content of 1 mM BSA by intensive mixing for 1 h. The Nalgene plastic test tubes with an inner diameter 9 mm containing 3 ml of the PVA solution with or without BSA were kept at Ϫ20°C for 2 h. After the freezing process, they were allowed to thaw and gradually warm up to 20°C for 24 h. Up to five freezing-thawing cycles were applied in the PVA cryogel preparation. Finally, the cryogels so produced were transferred from plastic tubes into 10-mm glass NMR tubes.…”

“…It was suggested from the macroscopic rigidity and melting temperature measurements that the crosslinking points of the aqueous PVA gel would present the syndiotactic sequences of six to eight monomer units (6,7). The aqueous gels of syndiotactic-rich PVA have melting temperatures and elastic moduli higher than those of atactic PVA aqueous gels (8).…”